Method of producing retardation film, method of producing circularly polarizing plate, and film-stretching apparatus, involving braking the moving speeds of film-holding clips

ABSTRACT

Provided is a method capable of producing a retardation film having an elongated shape and having a slow axis in an oblique direction with high production efficiency while precisely controlling the direction of the slow axis and the variation of the direction. The method of producing a retardation film of the present invention includes: holding the left and right end portions of a film with left and right variable pitch-type clips configured to have clip pitches changing in a longitudinal direction, respectively; preheating the film; obliquely stretching the film; and releasing the clips holding the film. The oblique stretching includes making the change ratio of the clip pitch of the clips on one side in the traveling direction thereof and the change ratio of the clip pitch of the clips on the other side in the traveling direction thereof different from each other, and/or making the position at which the clip pitch of the clips on the one side in the traveling direction starts to change and the position at which the clip pitch of the clips on the other side in the traveling direction starts to change different from each other, while extending a distance between the left and right clips. In the present invention, the moving speeds of the clips are braked immediately before the holding step and/or during a time period from the holding step to the oblique stretching step.

TECHNICAL FIELD

The present invention relates to a method of producing a retardation film and a method of producing a circularly polarizing plate, and a film-stretching apparatus.

BACKGROUND ART

A circularly polarizing plate has been used in an image display apparatus such as a liquid crystal display apparatus (LCD) or an organic electroluminescence display apparatus (OLED) for the purposes of improving its display characteristics and preventing reflection. The circularly polarizing plate is typically obtained by laminating a polarizer and a retardation film (typically a λ/4 plate) so that the absorption axis of the polarizer and the slow axis of the retardation film may form an angle of 45°. Hitherto, the retardation film has been typically produced by performing uniaxial stretching or biaxial stretching in a longitudinal direction and/or a lateral direction, and hence its slow axis is expressed in the lateral direction (widthwise direction) or longitudinal direction (lengthwise direction) of a raw film in many cases. As a result, in order to produce the circularly polarizing plate, it has been necessary to perform the following. The retardation film is cut so as to form an angle of 45° relative to its lateral direction or longitudinal direction, and the resultant pieces are bonded one by one.

To solve such problem, there has been proposed a technology involving performing stretching in an oblique direction to express the slow axis of the retardation film in the oblique direction (for example, Patent Literature 1). However, the stretching in the oblique direction involves the following problems: the direction of the slow axis of the retardation film to be obtained deviates from a preset value (typically becomes larger than the preset value) with time over a predetermined time period from the initial stage of the stretching; and even when the direction of the slow axis becomes constant after the predetermined time period, the variation of the direction periodically occurs along the lengthwise direction of the retardation film.

CITATION LIST Patent Literature

[PTL 1] JP 4845619 B2

SUMMARY OF INVENTION Technical Problem

The present invention has been made to solve the problems of the related art, and an object of the present invention is to provide a method capable of producing a retardation film having an elongated shape and having a slow axis in an oblique direction with high production efficiency while precisely controlling the direction of the slow axis and the variation of the direction. Another object of the present invention is to provide a method capable of producing a circularly polarizing plate excellent in optical characteristics with high production efficiency. Still another object of the present invention is to provide a film-stretching apparatus that can realize such production methods.

Solution to Problem

A method of producing a retardation film of the present invention includes holding left and right end portions of a film with left and right variable pitch-type clips configured to have clip pitches changing in a longitudinal direction, respectively, preheating the film, obliquely stretching the film, and releasing the clips holding the film. The oblique stretching includes “making a change ratio of the clip pitch of the clips on one side in a traveling direction thereof and a change ratio of the clip pitch of the clips on another side in a traveling direction thereof different from each other”, and/or, “making a position at which the clip pitch of the clips on the one side in the traveling direction starts to change and a position at which the clip pitch of the clips on the another side in the traveling direction starts to change different from each other”, while extending a distance between the left and right clips. In this method of producing a retardation film, moving speeds of the clips are braked immediately before the holding step and/or during a time period from the holding step to the oblique stretching step.

In one embodiment, the braking of the moving speeds of the clips includes applying a load to a link mechanism for changing the clip pitches to limit the clip moving speeds to predetermined speeds. In another embodiment, the braking of the moving speeds of the clips includes limiting the clip moving speeds to predetermined speeds through continuous or intermittent constant-speed rotation of an apparatus for braking the moving speeds of the clips.

In one embodiment, the braking of the moving speeds of the clips is performed at least once during a time period from a time point immediately before the holding step to the oblique stretching step, and is performed at least once during a time period from a time point immediately after the oblique stretching step to the releasing step.

In one embodiment, the oblique stretching step includes increasing the clip pitch of the clips on the one side and reducing the clip pitch of the clips on the another side, while extending the distance between the left and right clips, to obliquely stretch the film (first oblique stretching step), and maintaining or reducing the clip pitch of the clips on the one side and increasing the clip pitch of the clips on the another side so that the clip pitches of the left and right clips are equal to each other, while extending the distance between the left and right clips, to obliquely stretch the film (second oblique stretching step).

In one embodiment, in the first oblique stretching step, the reducing the clip pitch of the clips on the another side is started after the increasing the clip pitch of the clips on the one side is started.

In one embodiment, in the first oblique stretching step and the second oblique stretching step, an oblique stretching ratio S determined from the following equation (1) is 2.0 or more, and in the first oblique stretching step, a change ratio of the clip pitch of the clips on the another side is 0.5 or more and less than 1: S=√{square root over (W₃ ²+(v3′*(t3−t3′))²)}/W₁   Equation(1) where:

W₁ represents a width of the film before the first oblique stretching (unit: m);

W₃ represents a width of the film after the second oblique stretching (unit: m);

v3′ represents a moving speed of the clips having a clip pitch to be increased in the first oblique stretching step when the clip pitch of the clips changes to a predetermined clip pitch in the second oblique stretching step (unit: m/sec);

t3 represents a time period from entry of the clips having a clip pitch to be reduced in the first oblique stretching step into a preheating zone to an end of the second oblique stretching step (unit: sec); and

t3′ represents a time period from entry of the clips having a clip pitch to be increased in the first oblique stretching step into the preheating zone to the end of the second oblique stretching step (unit: sec).

In one embodiment, in the first oblique stretching step, a product of a change ratio of the clip pitch of the clips on the one side and a change ratio of the clip pitch of the clips on the another side is from 0.7 to 1.5.

In one embodiment, a formation material for the film contains a polycarbonate resin, a polyvinyl acetal resin, a cycloolefin-based resin, a cellulose-based resin, a cellulose ester-based resin, a polyester-based resin, or a polyester carbonate-based resin.

According to another aspect of the present invention, a retardation film is provided. The retardation film is produced by the production method above described, has an elongated shape, and has a slow axis in a direction forming a predetermined angle relative to a lengthwise direction.

According to still another aspect of the present invention, a method of producing a circularly polarizing plate is provided. The method of producing a circularly polarizing plate includes obtaining a retardation film having an elongated shape by the production method above described and continuously bonding the resultant retardation film having an elongated shape and a polarizing plate having an elongated shape with lengthwise directions thereof aligned with each other while conveying the film and the plate.

According to still another aspect of the present invention, a film-stretching apparatus is provided. The film-stretching apparatus includes left and right variable pitch-type clips that pass a preheating zone and an oblique stretching zone in the stated order while holding left and right end portions of a film to be stretched, and that each show a change in clip pitch in a longitudinal direction in association with running movement; and is configured to perform at least one selected from the following (i) and (ii), while extending a distance between the left and right clips, to obliquely stretch the film in the oblique stretching zone: (i) making a change ratio of the clip pitch of the clips on one side in a traveling direction thereof and a change ratio of the clip pitch of the clips on another side in a traveling direction thereof different from each other, and (ii) making a position at which the clip pitch of the clips on the one side in the traveling direction starts to change and a position at which the clip pitch of the clips on the another side in the traveling direction starts to change different from each other. This apparatus includes at least one means for braking moving speeds of the clips in a range of from a zone in which the film is held to the oblique stretching zone.

In one embodiment, the film-stretching apparatus includes at least one means for braking the moving speeds of the clips in the range of from the zone in which the film is held to the oblique stretching zone, and includes at least one means for braking the moving speeds of the clips in a range of from a position just behind the oblique stretching zone to a zone in which the clips holding the film are released.

Advantageous Effects of Invention

According to the embodiments of the present invention, the moving speeds of the clips are braked immediately before the holding step and/or during the time period from the holding step to the oblique stretching step in the method of producing a retardation film including oblique stretching, whereby the retardation film having an elongated shape and having a slow axis in an oblique direction can be produced with high production efficiency while the direction of the slow axis and the variation of the direction are precisely controlled. Further, according to the embodiments of the present invention, the retardation film thus obtained and the polarizing plate are laminated by the so-called roll-to-roll process, whereby the circularly polarizing plate excellent in optical characteristics can be obtained with high production efficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic plan view for illustrating the entire construction of an example of a stretching apparatus that can be used in a production method of the present invention.

FIG. 2 is a main portion schematic plan view for illustrating a link mechanism via which a clip pitch is changed in the stretching apparatus of FIG. 1, the view being an illustration of a state in which the clip pitch is minimum.

FIG. 3 is a main portion schematic plan view for illustrating the link mechanism via which the clip pitch is changed in the stretching apparatus of FIG. 1, the view being an illustration of a state in which the clip pitch is maximum.

FIG. 4 is a schematic view for illustrating oblique stretching in a production method according to one embodiment of the present invention.

FIG. 5 is a graph for showing a relationship between each zone of the stretching apparatus and the clip pitch at the time of the oblique stretching illustrated in FIG. 4.

FIG. 6 is a graph for showing a relationship between each zone of the stretching apparatus and the clip pitch at the time of the oblique stretching according to another embodiment.

FIG. 7 is a schematic view for illustrating oblique stretching in a production method according to another embodiment of the present invention.

FIG. 8 is a graph for showing a relationship between each zone of the stretching apparatus and the clip pitch at the time of the oblique stretching illustrated in FIG. 7.

FIG. 9 is a schematic view for illustrating a relationship between the oblique stretching in the production method of the present invention and the equation (1).

FIG. 10 are schematic views for illustrating the respective moving speeds of left and right clips in one embodiment of the oblique stretching in the production method of the present invention, and the equation (1).

FIG. 11 are schematic views for illustrating the respective moving speeds of left and right clips in another embodiment of the oblique stretching in the production method of the present invention, and the equation (1).

FIG. 12 is a schematic sectional view of a circularly polarizing plate using a retardation film obtained by the production method of the present invention.

FIG. 13 is a schematic view for illustrating a production method for a circularly polarizing plate according to one embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Now, preferred embodiments of the present invention are described. However, the present invention is not limited to these embodiments.

A method of producing a retardation film of the present invention includes: holding left and right end portions of a film with left and right variable pitch-type clips configured to have clip pitches changing in a longitudinal direction, respectively (holding step); preheating the film (preheating step); obliquely stretching the film (oblique stretching step); and releasing the clips holding the film (releasing step). The oblique stretching step includes making the change ratio of the clip pitch of the clips on one side in the traveling direction thereof and the change ratio of the clip pitch of the clips on the other side in the traveling direction thereof different from each other, and/or making the position at which the clip pitch of the clips on the one side in the traveling direction starts to change and the position at which the clip pitch of the clips on the other side in the traveling direction start s to change different from each other, while extending a distance between the left and right clips. In one embodiment, the oblique stretching step includes: increasing the clip pitch of the clips on one side and reducing the clip pitch of the clips on the other side, while extending the distance between the left and right clips, to obliquely stretch the film (first oblique stretching step); and maintaining or reducing the clip pitch of the clips on the one side and increasing the clip pitch of the clips on the other side so that the clip pitches of the left and right clips are equal to each other, while extending the distance between the left and right clips, to obliquely stretch the film (second oblique stretching step). In this description, each step in such embodiment including the first oblique stretching step and the second oblique stretching step as described above is described in detail. However, it is apparent to a person skilled in the art that the present invention is applicable to all methods each involving relatively changing the clip pitches of the left and right clips to obliquely stretch the film.

A. Holding Step

First, a stretching apparatus that can be used in the production method of the present invention including this step is described with reference to FIG. 1 to FIG. 3. FIG. 1 is a schematic plan view for illustrating the entire construction of an example of the stretching apparatus that can be used in the production method of the present invention. FIG. 2 and FIG. 3 are each a main portion schematic plan view for illustrating a link mechanism via which a clip pitch is changed in the stretching apparatus of FIG. 1, FIG. 2 being an illustration of a state in which the clip pitch is minimum and FIG. 3 being an illustration of a state in which the clip pitch is maximum. When viewed in plan view, a stretching apparatus 100 has, on both of its left and right sides, an endless loop 10L and an endless loop 10R each having many clips 20 for holding a film so that the loops may be bilaterally symmetric with each other. It should be noted that in this description, an endless loop on a left side when viewed from a film inlet side is referred to as “left endless loop 10L” and an endless loop on a right side is referred to as “right endless loop 10R”. Each of the clips 20 of the left and right endless loops 10L and 10R is guided by a reference rail 70 to cyclically move in a loop manner. The clips 20 of the left endless loop 10L cyclically move in a counterclockwise direction and the clips 20 of the right endless loop 10R cyclically move in a clockwise direction. In the stretching apparatus, a holding zone A, a preheating zone B, a first oblique stretching zone C, a second oblique stretching zone D, and a releasing zone E are arranged in the stated order from a sheet inlet side toward a sheet outlet side. It should be noted that those zones mean zones in which the film to be stretched is substantially held, preheated, subjected to first oblique stretching and second oblique stretching, and released, respectively, and do not mean mechanically or structurally independent sections. In addition, attention should be paid to the fact that a ratio among the lengths of the respective zones in the stretching apparatus of FIG. 1 is different from the actual length ratio.

In the holding zone A and the preheating zone B, the right and left endless loops 10R and 10L are configured to be substantially parallel to each other while being separated from each other by a distance corresponding to the initial width of the film to be stretched. In the first oblique stretching zone C and the second oblique stretching zone D, the right and left endless loops 10R and 10L are configured so that the distance by which the loops are separated from each other may gradually enlarge from the preheating zone B side toward the releasing zone E until the distance corresponds to the width of the film after its stretching. In the releasing zone E, the right and left endless loops 10R and 10L are configured to be substantially parallel to each other while being separated from each other by a distance corresponding to the width of the film after the stretching.

The clips (left clips) 20 of the left endless loop 10L and the clips (right clips) 20 of the right endless loop 10R can each independently cyclically move. For example, driving sprockets 11 and 12 of the left endless loop 10L are rotationally driven in the counterclockwise direction by electric motors 13 and 14, and the driving sprockets 11 and 12 of the right endless loop 10R are rotationally driven in the clockwise direction by the electric motors 13 and 14. As a result, a running force is imparted to a clip-carrying member 30 of each of drive rollers (not shown) engaging with the driving sprockets 11 and 12. Thus, the clips 20 of the left endless loop 10L cyclically move in the counterclockwise direction and the clips 20 of the right endless loop 10R cyclically move in the clockwise direction. The clips 20 of the left endless loop 10L and the clips 20 of the right endless loop 10R can each independently be cyclically moved by each independently driving a left electric motor and a right electric motor.

Further, the clips (left clips) 20 of the left endless loop 10L and the clips (right clips) 20 of the right endless loop 10R are each of a variable pitch type. That is, the clip pitches (clip-to-clip distances) of the left and right clips 20 and 20 in the traveling direction (that is, longitudinal direction: MD) can each independently change in association with their movement. The variable pitch type can be realized by any appropriate construction. Now, description is given by taking a link mechanism (pantograph mechanism) as an example.

As illustrated in FIG. 2 and FIG. 3, the elongated rectangular clip-carrying members 30 are arranged in a lateral direction in a planar view by which the clips 20 are individually carried. Although not shown, the clip-carrying members 30 are each formed so as to be of a frame structure closed by an upper beam, a lower beam, a front wall (wall on the clip side), and a rear wall (wall on a side opposite to the clip), and having a strong section. The clip-carrying members 30 are each arranged so as to roll on running road surfaces 81 and 82 by virtue of running wheels 38 on both of its ends. It should be noted that in FIG. 2 and FIG. 3, a running wheel on the front wall side (running wheel rolling on the running road surface 81) is not shown. The running road surfaces 81 and 82 are parallel to the reference rail 70 over an entire region. On the rear sides (sides opposite to the clip) of the upper beam and lower beam of each of the clip-carrying members 30, a long hole 31 is formed along the lengthwise direction of the clip-carrying member and a slider 32 engages slidably in the lengthwise direction of the long hole 31. One first axis member 33 is vertically arranged near an end portion of each of the clip-carrying members 30 on the clip 20 side so as to penetrate its upper beam and lower beam. Meanwhile, one second axis member 34 is vertically arranged so as to penetrate the slider 32 of each of the clip-carrying members 30. One end of a main link member 35 is pivotally linked to the first axis member 33 of each of the clip-carrying members 30. The other end of the main link member 35 is pivotally linked to the second axis member 34 of the adjacent clip-carrying member 30. In addition to the main link member 35, one end of a sub-link member 36 is pivotally linked to the first axis member 33 of each of the clip-carrying members 30. The other end of the sub-link member 36 is pivotally linked to the central portion of the main link member 35 by a pivot 37. By virtue of the link mechanism based on the main link member 35 and the sub-link member 36, as the extent to which the slider 32 moves toward the rear side of the clip-carrying member 30 (side opposite to the clip) becomes larger as illustrated in FIG. 2, a pitch between the clip-carrying members 30 in the longitudinal direction (hereinafter simply referred to as “clip pitch”) reduces, and as the extent to which the slider 32 moves toward the front side of the clip-carrying member 30 (clip side) becomes larger as illustrated in FIG. 3, the clip pitch increases. The positioning of the slider 32 is performed by a pitch-setting rail 90. As illustrated in FIG. 2 and FIG. 3, as the clip pitch becomes larger, the distance by which the reference rail 70 and the pitch-setting rail 90 are separated from each other reduces. It should be noted that additionally detailed description of the link mechanism is omitted because the mechanism is well-known in the art.

A retardation film having a slow axis in an oblique direction (e.g., a direction at 45° relative to the longitudinal direction) can be produced by obliquely stretching the film to be stretched with such stretching apparatus as described above. First, in the holding zone A (inlet of film intake by the stretching apparatus 100), both side edges of the film to be stretched are held with the clips 20 of the right and left endless loops 10R and 10L at constant clip pitches equal to each other, and the film is fed to the preheating zone B by the movement of the right and left endless loops 10R and 10L (substantially the movement of each of the clip-carrying members 30 guided by the reference rail 70).

In the embodiment of the present invention, at least one means 50 for braking the moving speeds of the clips is arranged in the range of from the holding zone A to the oblique stretching zone (the first oblique stretching zone C or the second oblique stretching zone D). In the example illustrated in FIG. 1, a pair of the braking means 50 is arranged in the left and right endless loops just in front of the preheating zone B. Typical examples of the braking means 50 include a high-torque sprocket and a constant-speed rotating sprocket. Arranging such braking means can prevent the direction of the slow axis of the retardation film to be obtained in the oblique stretching to be described later from deviating from a preset value (typically becoming larger than the preset value) with time over a predetermined time period from the initial stage of the stretching. More specifically, arranging such braking means can make the timing at which the clips enter the preheating zone constant. Therefore, the number of clips present in the preheating zone B and the oblique stretching zones (in this embodiment, the first oblique stretching zone C and the second oblique stretching zone D) can be controlled so as not to change. The foregoing results from the fact that arranging such braking means can prevent undesired movement of the clips resulting from a force in the oblique direction caused in the film by the oblique stretching, and play between a bearing supporting each clip and a rail. As a result, as described above, the deviation of the direction of the slow axis of the retardation film to be obtained in the oblique stretching with time can be prevented.

With regard to the operation of braking the moving speeds of the clips, a mode in which a load is applied to the link mechanism for changing the clip pitches is described. Description is given by taking the case where a high-torque sprocket is used as an example. Both the left and right clips that have held the film to be stretched in the holding zone A move in the holding zone A at predetermined moving speeds. Just in front of the preheating zone B, the high-torque sprocket 50 engages with the link mechanism for changing the clip pitches (substantially the axis member 34). The axis member 34 having engaged therewith the high-torque sprocket 50 receives the resistance of the sprocket to be braked (i.e., braking force is applied). As a result, the moving speeds of both the left and right clips entering the preheating zone can be controlled so as to be constant. Arranging the high-torque sprocket as described above synchronizes the left and right clips entering the preheating zone B with each other to prevent the deviation of the moving speeds and clip pitches of the left and right clips from preset values with time. Accordingly, the deviation of the direction of the slow axis of the retardation film to be obtained in the oblique stretching with time can be prevented.

Next, the case where braking means that continuously or intermittently rotates at a constant speed is used as an apparatus for braking the moving speeds of the clips is described. Description is given by taking the case where a constant-speed rotating sprocket (sprocket rotationally driven at a constant speed by an electric motor 51 as illustrated in the figure) is used as the braking means 50 as an example. Causing the sprocket 50 to rotate at a constant speed can not only brake the clips entering the preheating zone but also precisely control the speeds of the clips. More specifically, when the braking is performed with a high-torque sprocket (that is not rotationally driven), the deviation of the direction of the slow axis of the retardation film to be obtained in the oblique stretching with time can be prevented as described above, but the moving speeds of the clips entering the preheating zone show such a profile as to intermittently reduce. On the other hand, when the sprocket that rotates at a constant speed is used, the moving speeds of the clips entering the preheating zone show a smooth profile from which such intermittent reduction portion is removed. As a result, in addition to a preventing effect on the deviation of the direction of the slow axis of the retardation film to be obtained, the variation of the direction of the slow axis of the retardation film periodically occurring along the lengthwise direction of the film can be significantly suppressed.

In this description, the embodiment in which the braking means 50 is arranged just in front of the preheating zone as described above is described. However, the braking means 50 is arranged at any appropriate position as long as the position falls within the range of from the holding zone to the oblique stretching zone. It is apparent that when the braking means 50 is arranged in the range of from the holding zone to the oblique stretching zone, the above-mentioned effect is obtained irrespective of the position at which the means is arranged. For example, the braking means 50 may be arranged just in front of the holding zone, may be arranged at any position in the holding zone, may be arranged at any position in the preheating zone, may be arranged just in front of the oblique stretching zone, or may be arranged in the oblique stretching zone. For example, when the braking means 50 is arranged in the oblique stretching zone, the variations of the moving speeds of the clips caused by tension at the time of the stretching can be suppressed. As a result, the variations of the optical characteristics of the retardation film can be suppressed. Further, the plurality of braking means 50 may be arranged in the range of from the holding zone A to the releasing zone E (e.g., in the range of from the holding zone to the oblique stretching zone). For example, in the embodiment illustrated in FIG. 1, another braking means may be further arranged in the oblique stretching zone (the first oblique stretching zone C and/or the second oblique stretching zone D), another braking means may be further arranged in a thermal fixing zone (not shown) between the oblique stretching zone and the releasing zone, and/or another braking means may be further arranged in the releasing zone E (the other braking means is not shown). The position at which the braking means is arranged, the number of the means to be arranged, and a specific form of the means can be appropriately set depending on, for example, purposes and the size of a stretching apparatus (typically, the length of each zone and the distance by which the loops are separated from each other). It should be noted that a pair of the braking means can be typically arranged as a unit in the left and right endless loops. In one embodiment, at least one (typically, at least a pair of) braking means is arranged in the range of from the holding zone A (e.g., a position immediately before the holding of the film in the holding zone) to the oblique stretching zone (substantially the second oblique stretching zone D), and at least one (typically, at least a pair of) braking means is arranged in the range of from a position just behind the oblique stretching zone (substantially the second oblique stretching zone D) to the releasing zone E.

B. Preheating Step

In the preheating zone (preheating step) B, as described above, the right and left endless loops 10R and 10L are configured to be substantially parallel to each other while being separated from each other by a distance corresponding to the initial width of the film to be stretched, and hence the film is basically heated without being laterally stretched or longitudinally stretched. However, a distance between the left and right clips (distance in a widthwise direction) maybe slightly widened in order to avoid, for example, the following inconvenience: the film sags owing to the preheating to be brought into contact with a nozzle in an oven.

In the preheating step, the film is heated to a temperature T1 (° C.). The temperature T1 is preferably equal to or more than the glass transition temperature (Tg) of the film, more preferably equal to or more than Tg+2° C., still more preferably equal to or more than Tg+5° C. Meanwhile, the heating temperature T1 is preferably equal to or less than Tg+40° C., more preferably equal to or less than Tg+30° C. The temperature T1 is, for example, from 70° C. to 190° C., preferably from 80° C. to 180° C., though the temperature varies depending on the film to be used.

A time period required for the temperature of the film to be increased to the temperature T1 and a time period for which the temperature is held at the temperature T1 can be appropriately set depending on a constituent material for the film and a production condition (e.g., the speed at which the film is conveyed). The temperature increase time period and the holding time period can be controlled by adjusting, for example, the moving speeds of the clips 20, the length of the preheating zone, and the temperature of the preheating zone.

C. First Oblique Stretching Step

In the first oblique stretching zone (first oblique stretching step) C, the film is obliquely stretched by increasing the clip pitch of the clips on one side and reducing the clip pitch of the clips on the other side while extending the distance between the left and right clips (more specifically, the distance by which the right and left endless loops 10R and 10L are separated from each other). When the clip pitches are changed as described above, the left and right clips are moved at different speeds, whereby the oblique stretching can be performed while one side edge portion of the film is stretched in its lengthwise direction and the other side edge portion of the film is shrunk in the lengthwise direction. As a result, a slow axis can be expressed in a desired direction (e.g., a direction at 45° relative to the lengthwise direction) with high uniaxiality and a high in-plane alignment property.

One embodiment of the first oblique stretching is hereinafter specifically described with reference to FIG. 4 and FIG. 5. First, in the preheating zone B, both the left and right clip pitches are set to P₁. P₁ represents a clip pitch upon holding of the film. Next, simultaneously with the entry of the film into the first oblique stretching zone C, the increase of the clip pitch of the clips on one side (right side in the illustrated example) is started and the reduction of the clip pitch of the clips on the other side (left side in the illustrated example) is started. In the first oblique stretching zone C, the clip pitch of the right clips is increased to P₂ and the clip pitch of the left clips is reduced to P₃. Therefore, in the terminating portion of the first oblique stretching zone C (starting portion of the second oblique stretching zone D), the left clips move at the clip pitch P₃ and the right clips move at the clip pitch P₂. It should be noted that a ratio between the clip pitches can generally correspond to a ratio between the moving speeds of the clips. Accordingly, the ratio between the clip pitches of the left and right clips can generally correspond to a ratio between the stretching ratios of the right side edge portion and left side edge portion of the film in the MD direction.

In FIG. 4 and FIG. 5, both the position at which the clip pitch of the right clips starts to increase and the position at which the clip pitch of the left clips starts to reduce are each defined as the starting portion of the first oblique stretching zone C. However, unlike the illustrated example, the clip pitch of the left clips may start to reduce after the clip pitch of the right clips has started to increase (e.g., FIG. 6), or the clip pitch of the right clips may start to increase after the clip pitch of the left clips has started to reduce (not shown). In one preferred embodiment, after the clip pitch of the clips on one side has started to increase, the clip pitch of the clips on the other side starts to reduce. According to such embodiment, the film has already been stretched in the widthwise direction to a certain extent (preferably from about 1.2 times to 2.0 times), and hence a wrinkle hardly occurs even when the clip pitch on the other side is largely reduced. Accordingly, the oblique stretching can be performed at an additionally acute angle, and hence a retardation film having high uniaxiality and a high in-plane alignment property can be suitably obtained.

Similarly, in FIG. 4 and FIG. 5, the increase of the clip pitch of the right clips and the reduction of the clip pitch of the left clips continue up to the terminating portion of the first oblique stretching zone C (starting portion of the second oblique stretching zone D), but unlike the illustrated example, the following is permitted: one of the increase and reduction of the clip pitches ends before the terminating portion of the first oblique stretching zone C, and the clip pitch is maintained as it is up to the terminating portion of the first oblique stretching zone C.

The change ratio (P₂/ P₁) of the clip pitch to be increased is preferably from 1.25 to 1.75, more preferably from 1.30 to 1.70, still more preferably from 1.35 to 1.65. In addition, the change ratio (P₃/P₁) of the clip pitch to be reduced is, for example, 0.50 or more and less than 1, preferably from 0.50 to 0.95, more preferably from 0.55 to 0.90, still more preferably from 0.55 to 0.85. When the change ratios of the clip pitches fall within such ranges, a slow axis can be expressed in a direction at about 45° relative to the lengthwise direction of the film with high uniaxiality and a high in-plane alignment property.

As described above, the clip pitches can be adjusted by positioning the sliders through the adjustment of the distance by which the pitch-setting rail and reference rail of the stretching apparatus are separated from each other.

The stretching ratio (W₂/W₁) of the film in the widthwise direction in the first oblique stretching step is preferably from 1.1 times to 3.0 times, more preferably from 1.2 times to 2.5 times, still more preferably from 1.25 times to 2.0 times. When the stretching ratio is less than 1.1 times, a corrugated galvanized iron-like wrinkle may occur in the side edge portion on the shrunk side. In addition, when the stretching ratio is more than 3.0 times, the biaxiality of the retardation film to be obtained is raised, and hence in the case where the film is applied to a circularly polarizing plate or the like, its viewing angle characteristic may reduce.

In one embodiment, the first oblique stretching is performed so that the product of the change ratio of the clip pitch of the clips on one side and the change ratio of the clip pitch of the clips on the other side may be preferably from 0.7 to 1.5, more preferably from 0.8 to 1.45, still more preferably from 0.85 to 1.40. When the product of the change ratios falls within such range, a retardation film having high uniaxiality and a high in-plane alignment property can be obtained.

The first oblique stretching can be typically performed at a temperature T2. The temperature T2 is preferably from Tg−2° C. to Tg+30° C. where Tg represents the glass transition temperature of the resin film, more preferably from Tg−10° C. to Tg+20° C., particularly preferably about Tg. The temperature T2 is, for example, from 70° C. to 180° C., preferably from 80° C. to 170° C., though the temperature varies depending on the resin film to be used. A difference (T1−T2) between the temperature T1 and the temperature T2 is preferably ±2° C. or more, more preferably ±5° C. or more. In one embodiment, the relationship between T1 and T2 satisfies T1>T2 and hence the film heated to the temperature T1 in the preheating step can be cooled to the temperature T2.

D. Second Oblique Stretching Step

In the second oblique stretching zone (second oblique stretching step) D, the film is obliquely stretched by maintaining or reducing the clip pitch of the clips on the one side and increasing the clip pitch of the clips on the other side so that the clip pitches of the left and right clips are equal to each other, while extending the distance between the left and right clips (more specifically, the distance by which the right and left endless loops 10R and 10L are separated from each other). When the oblique stretching is performed while the difference between the left and right clip pitches is reduced as described above, the film can be sufficiently stretched in an oblique direction while an excess stress is alleviated. In addition, the film can be subjected to the releasing step in a state where the moving speeds of the left and right clips are equal to each other, and hence a variation in, for example, speed at which the film is conveyed hardly occurs at the time of the release of the left and right clips, and subsequent take-up of the film can be suitably performed.

One embodiment of the second oblique stretching is hereinafter specifically described with reference to FIG. 4 and FIG. 5. First, the increase of the clip pitch of the left clips is started simultaneously with the entry of the film into the second oblique stretching zone D. In the second oblique stretching zone D, the clip pitch of the left clips is increased to P₂. Meanwhile, the clip pitch of the right clips is maintained at P₂ in the second oblique stretching zone D. Therefore, in the terminating portion of the second oblique stretching zone D (starting portion of the releasing zone E) both the left clips and the right clips move at the clip pitch P₂.

The change ratio (P₂/P₃) of the clip pitch to be increased in the embodiment is not limited as long as the effects of the present invention are not impaired. The change ratio (P₂/P₃) is, for example, from 1.3 to 4.0, preferably from 1.5 to 3.0.

Next, another embodiment of the second oblique stretching is specifically described with reference to FIG. 7 and FIG. 8. First, simultaneously with the entry of the film into the second oblique stretching zone D, the reduction of the clip pitch of the right clips is started and the increase of the clip pitch of the left clips is started. In the second oblique stretching zone D, the clip pitch of the right clips is reduced to P₄ and the clip pitch of the left clips is increased to P₄. Therefore, in the terminating portion of the second oblique stretching zone D (starting portion of the releasing zone E), both the left clips and the right clips move at the clip pitch P₄. It should be noted that in the illustrated example, for simplicity, both the position at which the clip pitch of the right clips starts to reduce and the position at which the clip pitch of the left clips starts to increase are each de fined as the starting portion of the second oblique stretching zone D, but these positions maybe different positions. Similarly, the position at which the reduction of the clip pitch of the right clips ends and the position at which the increase of the clip pitch of the left clips ends may be different positions.

The change ratio (P₄/P₂) of the clip pitch to be reduced in the embodiment and the change ratio (P₄/P₃) of the clip pitch to be increased therein are not limited as long as the effects of the present invention are not impaired. The change ratio (P₄/P₂) is, for example, 0.4 or more and less than 1.0, preferably from 0.6 to 0.95. In addition, the change ratio (P₄/P₃) is, for example, more than 1.0 and 2.0 or less, preferably from 1.2 to 1.8. The P₄ is preferably equal to or more than the P₁. When P₄<P₁, a problem such as the occurrence of a wrinkle in a side end portion or an increase in biaxiality may occur.

The stretching ratio (W₃/W₂) of the film in the widthwise direction in the second oblique stretching step is preferably from 1.1 times to 3.0 times, more preferably from 1.2 times to 2.5 times, still more preferably from 1.25 times to 2.0 times. When the stretching ratio is less than 1.1 times, a corrugated galvanized iron-like wrinkle may occur in the side edge portion on the shrunk side. In addition, when the stretching ratio is more than 3.0 times, the biaxiality of the retardation film to be obtained is raised, and hence in the case where the film is applied to a circularly polarizing plate or the like, its viewing angle characteristic may reduce. In addition, a stretching ratio (W₃/W₁) in the widthwise direction in the first oblique stretching step and the second oblique stretching step is preferably from 1.2 times to 4.0 times, more preferably from 1.4 times to 3.0 times from the same viewpoints as those described above.

In one embodiment, the first oblique stretching and the second oblique stretching are performed so that an oblique stretching ratio determined from the following equation (1) may be preferably 2.0 or more, more preferably from 2.0 to 4.0, still more preferably from 2.5 to 3.5. When the oblique stretching ratio is less than 2.0, the biaxiality of the retardation film may be raised or its in-plane alignment property may reduce. S=√{square root over (W₃ ²+(v3′*(t3−t3′))²)}/W₁   Equation(1) (In the equation:

W₁ represents the width of the film before the first oblique stretching;

W₃ represents the width of the film after the second oblique stretching;

v3′ represents the moving speed of the clips having a clip pitch to be increased in the first oblique stretching step when the clip pitch of the clips changes to a predetermined clip pitch in the second oblique stretching step;

t3 represents a time period from the entry of the clips having a clip pitch to be reduced in the first oblique stretching step into the preheating zone to the end of the second oblique stretching step; and

t3′ represents a time period from the entry of the clips having a clip pitch to be increased in the first oblique stretching step into the preheating zone to the end of the second oblique stretching step.)

With regard to the v3′, the predetermined clip pitch means a clip pitch after the maintenance or reduction of the clip pitch in the second oblique stretching step following the completion of the increase of the clip pitch in the first oblique stretching step, and corresponds to the P₂ or P₄ in the description of the section C. In addition, when the moving speed of the clips having a clip pitch to be increased in the first oblique stretching step at a time when the clip pitch of the clips is changed to the predetermined clip pitch (corresponding to the P₂ in the description of the section C) in the first oblique stretching step is represented by v2′, in the case where v2′=v3′, the t3 is represented by the following equation (2) and the t3′ is represented by the following equation (3), and in the case where v2′>v3′, the t3 is represented by the following equation (4) and the t3′ is represented by the following equation (5).

The equations (2) to (4) are hereinafter described. In the description of the respective symbols in the equations, reference can be made to FIGS. 9 to 11. It should be noted that an asterisk mark (*) in each of the equations (1) to (5) is a multiplication sign. In addition, the unit of a film width is m, the unit of a velocity is m/sec, the unit of a distance is m, and the unit of a time period is sec. t3=(1/a1)*ln(a1*L3+b1)−(1/a1)*ln(a1*L2+b1)+(1/a)*ln(a*L2+b) −(1/a)*ln(a*L1+b)+L1/v1  Equation(2) (In the equation: a1=(v2−v3)/(L2−L3); b1=v3−a1*L3; a=(v1−v2)/(L1−L2); b=v2−a*L2;

v1 represents the moving speed of the clips having a clip pitch to be reduced in the first oblique stretching step when the clips pass the preheating zone;

v2 represents the moving speed of the clips having a clip pitch to be reduced in the first oblique stretching step when the clip pitch of the clips is reduced to a predetermined clip pitch (corresponding to the P₃ in the description of the section C) in the first oblique stretching step;

v3 represents the moving speed of the clips having a clip pitch to be reduced in the first oblique stretching step when the clip pitch of the clips is increased to a predetermined clip pitch (corresponding to the P₂ or P₄ in the description of the section D) in the second oblique stretching step;

L1 represents a distance from the inlet of the preheating zone to the position at which the clips having a clip pitch to be reduced in the first oblique stretching step start to reduce the clip pitch (in one embodiment, a distance from the inlet of the preheating zone to the outlet of the preheating zone);

L2 represents a distance from the inlet of the preheating zone to the position at which the clips having a clip pitch to be reduced in the first oblique stretching step start to increase the clip pitch (in one embodiment, a distance from the inlet of the preheating zone to the outlet of the first oblique stretching zone); and

L3 represents a distance from the inlet of the preheating zone to the position at which the clips having a clip pitch to be reduced in the first oblique stretching step end the increase of the clip pitch (in one embodiment, a distance from the inlet of the preheating zone to the outlet of the second oblique stretching zone).) t3′=(L1′/v1′)+(1/a′)*ln(a′*L2′+b′)−(1/a′)*ln(a′*L1′+b′)+(L3′−L2′)/v3′  Equation(3) (In the equation: a′=(v1′−v2′)/(L1′−L2′); b′=v3′−a′*L2′;

v1′ represents the moving speed of the clips having a clip pitch to be increased in the first oblique stretching step when the clips pass the preheating zone;

v2′ represents the moving speed of the clips having a clip pitch to be increased in the first oblique stretching step when the clip pitch of the clips is increased to a predetermined clip pitch (corresponding to the P₂ in the description of the section C) in the first oblique stretching step,

v3′ represents the moving speed of the clips having a clip pitch to be increased in the first oblique stretching step when the clips pass the second oblique stretching zone;

L1′ represents a distance from the inlet of the preheating zone to the position at which the clips having a clip pitch to he increased in the first oblique stretching step start to increase the clip pitch (in one embodiment, a distance from the inlet of the preheating one to the outlet of the preheating zone);

L2′ represents a distance from the inlet of the preheating zone to the position at which the clips having a clip pitch to be increased in the first oblique stretching step end the increase of the clip pitch (in one embodiment, a distance from the inlet of the preheating zone to the outlet of the first oblique stretching zone); and

L3′ represents a distance from the inlet of the preheating zone to the outlet of the second oblique stretching zone.) t3(1/a1)*ln(a1*L3+b1)−(1/a1)*ln(a1*L2+b1)+(1/a1)*ln(a*L2+b)−(1/a)*ln(a*L1+b)+L1/v1   Equation(4) (In the equation, a1, b1, a, b, v1, v2, v3, L1, L2, and L3 are as defined for the equation (2).) t3′=(L1′/v1′)+(1/a′)*ln(a′*L2′+b′)−(1/a′)*ln(a′*L1′+b′)+(1/a″)*ln(a″*L3′+b″)−(1/a″)*ln(a″*L2′+b″)   Equation(5) (In the equation: a′=(v1′−v2′)/(L1′−L2′); b′=v2′−a′*L2′; a″=(v2′−v3′)/(L2′−L3′); b″=v3′−a″*L3′;

v1′ represents the moving speed of the clips having a clip pitch to be increased in the first oblique stretching step when the clips pass the preheating zone;

v2′ represents the moving speed of the clips having a clip pitch to be increased in the first oblique stretching step when the clip pitch of the clips is increased to a predetermined clip pitch (corresponding to the P₂ in the description of the section C) in the first oblique stretching step;

v3′ represents the moving speed of the clips having a clip pitch to be increased in the first oblique stretching step when the clip pitch of the clips is reduced to a predetermined clip pitch (corresponding to the P₄ in the description of the section D) in the second oblique stretching step;

L1′ represents a distance from the inlet of the preheating zone to the position at which the clips having a clip pitch to be increased in the first oblique stretching step start to increase the clip pitch (in one embodiment, a distance from the inlet of the preheating zone to the outlet of the preheating zone);

L2′ represents a distance from the inlet of the preheating zone to the position at which the clips having a clip pitch to be increased in the first oblique stretching step end the increase of the clip pitch (in one embodiment, a distance from the inlet of the preheating zone to the outlet of the first oblique stretching zone); and

L3′ represents a distance from the inlet of the preheating zone to the position at which the clips having a clip pitch to be increased in the first oblique stretching step end the reduction of the clip pitch to a predetermined clip pitch (corresponding to the P₄ in the description of the section D) in the second oblique stretching step (in one embodiment, a distance from the inlet of the preheating zone to the outlet of the second oblique stretching zone).)

The second oblique stretching can be typically performed at a temperature T3. The temperature T3 can be equal to the temperature T2.

E. Releasing Step

Finally, the clips holding the film are released, whereby the retardation film is obtained. The clips are released after the following has been performed as required: the stretched state of the film is fixed by subjecting the film to heat treatment (thermal fixing step), and the film is cooled.

The heat treatment can be typically performed at a temperature T4. The temperature T4 varies depending on the film to be stretched. In some cases, T3≥T4, and in other cases, T3<T4. In general, when the film is formed of an amorphous material, T3≥T4, and when the film is formed of a crystalline material, a crystallization treatment may be performed by setting the T3 so that the T3 maybe lower than the T4. When T3≥T4, a difference (T3−T4) between the temperatures T3 and T4 is preferably from 0° C. to 50° C. A heat treatment time is typically from 10 seconds to 10 minutes.

The thermally fixed film is cooled to Tg or less in ordinary cases. After that, the clips have been released, and then, portions held by the clips on both ends of the film are cut, and the remainder is taken up.

F. Film to be Stretched and Retardation Film Obtained by Stretching

The film to be suitably used in the production method of the present invention (substantially the stretching method described in the section A to the section E) is, for example, any appropriate film that can be used as a retardation film. As a constituent material for the film, there are given, for example, a polycarbonate resin, a polyvinyl acetal resin, a cycloolefin-based resin, an acrylic resin, a cellulose ester-based resin, a cellulose-based resin, a polyester-based resin, a polyester carbonate-based resin, an olefin-based resin, and a polyurethane-based resin. Of those, a polycarbonate resin, a polyvinyl acetal resin, a cellulose ester-based resin, a polyester-based resin, or a polyester carbonate-based resin is preferred because a retardation film showing so-called reverse wavelength dispersion dependency can be obtained with any one of these resins. Those resins may be used alone or in combination depending on desired characteristics.

Any appropriate polycarbonate-based resin is used as the polycarbonate-based resin. A preferred example thereof is a polycarbonate resin containing a structural unit derived from a dihydroxy compound. Specific examples of the dihydroxy compound include 9,9-bis(4-hydroxyphenyl)fluorene, 9,9-bis(4-hydroxy-3-methylphenyl)fluorene, 9,9-bis(4-hydroxy-3-ethylphenyl)fluorene, 9,9-bis(4-hydroxy-3-n-propylphenyl)fluorene, 9,9-bis(4-hydroxy-3-isopropylphenyl)fluorene, 9,9-bis(4-hydroxy-3-n-butylphenyl)fluorene, 9,9-bis(4-hydroxy-3-sec-butylphenyl)fluorene, 9,9-bis(4-hydroxy-3-tert-butylphenyl)fluorene, 9,9-bis(4-hydroxy-3-cyclohexylphenyl)fluorene, 9,9-bis(4-hydroxy-3-phenylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)phenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3-methylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3-isopropylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3-isobutylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3-tert-butylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3-cyclohexylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3-phenylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3,5-dimethylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3-tert-butyl-6-methylphenyl)fluorene, and 9,9-bis(4-(3-hydroxy-2,2-dimethylpropoxy)phenyl)fluorene. The polycarbonate resin may contain a structural unit derived from the dihydroxy compound as well as a structural unit derived from a dihydroxy compound such as isosorbide, isomannide, isoidide, spiroglycol, dioxaneglycol, diethylene glycol (DEG), triethylene glycol (TEG), polyethylene glycol (PEG), or a bisphenol.

The polycarbonate resin as described above is disclosed in, for example, JP 2012-67300 A and JP 3325560 B2 in detail. The disclosures of the patent literatures are incorporated herein by reference.

The glass transition temperature of the polycarbonate resin is preferably 110° C. or more and 250° C. or less, more preferably 120° C. or more and 230° C. or less. When the glass transition temperature is excessively low, the heat resistance of the resin tends to deteriorate and hence the resin may cause a dimensional change after its forming into a film. When the glass transition temperature is excessively high, the forming stability of the resin at the time of its forming into a film may deteriorate. In addition, the transparency of the film may be impaired. It should be noted that the glass transition temperature is determined in conformity with JIS K 7121 (1987).

Any appropriate polyvinyl acetal resin may be used as the polyvinyl acetal resin. The polyvinyl acetal resin can be typically obtained by subjecting at least two kinds of aldehyde compounds and/or ketone compounds, and a polyvinyl alcohol-based resin to a condensation reaction. Specific examples of the polyvinyl acetal resin and a detailed production method therefor are disclosed in, for example, JP 2007-161994 A. The disclosure is incorporated herein by reference.

The refractive index characteristics of the retardation film obtained by stretching the film to be stretched preferably show a relationship of nx>ny. In addition, the in-plane alignment property of the retardation film is preferably high, and for example, its birefringent index Δn (Δn=nx−ny) measured at a wavelength of 550 nm is preferably from 0.002 to 0.005, more preferably from 0.0025 to 0.004. Further, the retardation film can preferably function as a λ/4 plate. An in-plane retardation Re (550) of the retardation film is preferably from 100 nm to 180 nm, more preferably from 135 nm to 155 nm. It should be noted that in this description, nx represents a refractive index in a direction in which an in-plane refractive index becomes maximum (i.e., a slow axis direction), ny represents a refractive index in a direction perpendicular to the slow axis in a plane (i.e., a fast axis direction), and nz represents a thickness direction refractive index. In addition, Re(λ) represents the in-plane retardation of the film measured with light having a wavelength of λ nm at 23° C. Therefore, the Re (550) represents the in-plane retardation of the film measured with light having a wavelength of 550 nm at 23° C. The Re(λ) is determined from the equation “Re(λ)−(nx−ny)×d” where d represents the thickness (nm) of the film.

The retardation film shows any appropriate refractive index ellipsoid as long as the ellipsoid has a relationship of nx>ny. The refractive index ellipsoid of the retardation film preferably shows a relationship of nx>ny≥nz. The Nz coefficient of the retardation film is preferably from 1 to 1.3, more preferably from 1 to 1.25, still more preferably from 1 to 1.2. The Nz coefficient is determined from Nz=Rth(λ)/Re(λ), where Rth(λ) represents the thickness direction retardation of the film measured with light having a wavelength of λ nm at 23° C. Rth(λ) is determined from the equation “Rth(λ)=(nx−nr)×d”.

The retardation film preferably shows so-called reverse wavelength dispersion dependency. Specifically, the in-plane retardation thereof satisfies a relationship of Re(450)<Re(550)<Re(650). Re(450)/Re(550) is preferably 0.8 or more and less than 1.0, more preferably from 0.8 to 0.95. Re(550)/Re(650) is preferably 0.8 or more and less than 1.0, more preferably from 0.8 to 0.97.

The retardation film has an absolute value of its photoelastic coefficient of preferably from 2×10⁻¹² (m²/N) to 100×10⁻¹² (m²/N), more preferably from 2×10⁻¹² (m²/N) to 50×10⁻¹² (m²/N)

G. Circularly Polarizing Plate and Production Method for Circularly Polarizing Plate

Typically, the retardation film obtained by the production method of the present invention can be suitably used in a circularly polarizing plate. FIG. 12 is a schematic sectional view of an example of such circularly polarizing plate. A circularly polarizing plate 300 of the illustrated example includes a polarizer 310, a first protective film 320 arranged on one side of the polarizer 310, a second protective film 330 arranged on the other side of the polarizer 310, and a retardation film 340 arranged outside the second protective film 330. The retardation film 340 is the retardation film obtained by the production method of the present invention. The second protective film 330 may be omitted. In that case, the retardation film 340 can function as a protective film for the polarizer. An angle formed between the absorption axis of the polarizer 310 and the slow axis of the retardation film 340 is preferably from 30° to 60°, more preferably from 38° to 52°, still more preferably from 43° to 47°, particularly preferably about 45°. It should be noted that detailed description of the constructions of the polarizer and the protective film is omitted because the constructions are well-known in the art.

The circularly polarizing plate may further include any appropriate optical member or optical functional layer at any appropriate position depending on purposes. For example, the outer surface of the first protective film 320 maybe subjected to a surface treatment such as a hard coat treatment, an antireflection treatment, an antisticking treatment, an antiglare treatment, or a light diffusion treatment. In addition, another retardation film showing any appropriate refractive index ellipsoid may be arranged on at least one side of the retardation film 340 depending on purposes. Further, an optical member such as a front substrate (e.g., a transparent protective substrate or a touch panel) may be arranged outside the first protective film 320.

The retardation film obtained by the production method of the present invention is extremely suitable for the production of a circularly polarizing plate. Details about the foregoing are as described below. The retardation film has an elongated shape and has a slow axis in an oblique direction (as described above, a direction at, for example, 45° relative to its lengthwise direction). In many cases, a polarizer having an elongated shape has an absorption axis in its lengthwise direction or widthwise direction, and hence the use of the retardation film obtained by the production method of the present invention enables the utilization of the so-called roll-to-roll process and enables the production of a circularly polarizing plate with extremely excellent production efficiency. Moreover, the retardation film obtained by the production method of the present invention has high uniaxiality and a high in-plane alignment property, and hence can provide a circularly polarizing plate having extremely excellent optical characteristics. It should be noted that the roll-to-roll process refers to a method involving continuously bonding films each having an elongated shape with their lengthwise directions aligned with each other while conveying the films with a roll.

A production method for a circularly polarizing plate according to one embodiment of the present invention is simply described with reference to FIG. 13. In FIG. 13, reference symbols 811 and 812 represent rolls for taking up a polarizing plate and a retardation film, respectively, and reference symbol 822 represents a conveying roll. In the illustrated example, the polarizing plate (first protective film 320/polarizer 310/second protective film 330) and the retardation film 340 are fed in a direction indicated by the arrow, and are bonded under a state in which their respective lengthwise directions are aligned with each other. At that time, the bonding is performed so that the second protective film 330 of the polarizing plate and the retardation film 340 are adjacent to each other. Thus, such circularly polarizing plate 300 as illustrated in FIG. 12 can be obtained. Although not shown, a circularly polarizing plate in which the retardation film 340 functions as a protective film can also be produced by, for example, bonding the polarizing plate (first protective film 320/polarizer 310) and the retardation film 340 so that the polarizer 310 and the retardation film 340 are adjacent to each other.

EXAMPLES

Now, the present invention is specifically described by way of Examples. However, the present invention is not limited by Examples below. It should be noted that measurement and evaluation methods in Examples are as described below.

(1) Alignment Angle (Direction in which Slow Axis is Expressed)

A sample was produced by cutting the central portion of a retardation film obtained in each of Examples and Comparative Example into a square shape measuring 50 mm wide by 50 mm long so that one side of the square was parallel to the widthwise direction of the film. An alignment angle θ of the sample at a wavelength of 590 nm was measured with an on-line phase difference meter (manufactured by Oji Scientific Instruments Co., Ltd., product name: “KOBRA-WI”). The alignment angle e was measured for each of the retardation films obtained immediately after the initiation of the stretching and the retardation films obtained over a time period of 1 hour after the initiation of the stretching. It should be noted that the alignment angle θ was measured under a state in which the sample was placed so as to be parallel to a measuring stage.

(2) Variation of Alignment Angle

The alignment angle of each of the retardation films obtained in Examples and Comparative Example at a central portion in its widthwise direction was measured during a time period from 1 hour after the initiation of the stretching to 1 hour 10 minutes thereafter, and the variation of the alignment angle was measured.

(3) Thickness

Measurement was performed with a microgauge-type thickness meter (manufactured by Mitutoyo Corporation).

Example 1

(Production of Polycarbonate Resin Film)

Polymerization was performed with a batch polymerization apparatus formed of two vertical reactors each including a stirring blade and a reflux condenser controlled to 100° C. 9,9-[4-(2-Hydroxyethoxy)phenyl]fluorene (BHEPF), isosorbide (ISB), diethylene glycol (DEG), diphenyl carbonate (DPC), and magnesium acetate tetrahydrate were loaded at a molar ratio “BHEPF/ISB/DEG/DPC/magnesium acetate” of 0.348/0.490/0.162/1.005/1.00×10⁻⁵. After a first reactor had been sufficiently purged with nitrogen (oxygen concentration: 0.0005 vol % to 0.001 vol %), the inside of the reactor was warmed with a heating medium, and when a temperature in the reactor reached 100° C., stirring was started. 40 Minutes after the start of the temperature increase, the internal temperature was caused to reach 220° C. and the reactor was controlled so as to hold the temperature, and at the same time, a pressure reduction was started. 90 Minutes after the temperature had reached 220° C., a pressure in the reactor was set to 13.3 kPa. A phenol vapor produced as a by-product of the polymerization reaction was introduced into the reflux condenser at 100° C., a monomer component present in a slight amount in the phenol vapor was returned to the reactor, and a phenol vapor that did not condense was introduced into a condenser at 45° C. and recovered.

Nitrogen was introduced into the first reactor to return the pressure to the atmospheric pressure once. After that, an oligomerized reaction liquid in the first reactor was transferred to a second reactor. Next, the increase of a temperature in the second reactor and the reduction of a pressure therein were started, and the internal temperature and the pressure were set to 240° C. and 0.2 kPa, respectively in 50 minutes. After that, the polymerization was caused to proceed until predetermined stirring power was achieved. When the predetermined power was achieved, nitrogen was introduced into the reactor to return the pressure to the atmospheric pressure, and the reaction liquid was taken out in the form of a strand and pelletized with a rotary cutter. Thus, a polycarbonate resin A having a copolymerization composition “BHEPF/ISB/DFG” of 34.8/49.0/16.2 [mol %] was obtained. The polycarbonate resin had a reduced viscosity of 0.430 dL/g and a glass transition temperature of 128° C.

The resultant polycarbonate resin was dried in a vacuum at 80° C. for 5 hours, and was then formed into a polycarbonate resin film having a thickness of 150 μm with a film-producing apparatus including a uniaxial extruder (manufactured by Isuzu Kakoki, screw diameter: 25 mm, cylinder preset temperature: 220° C.), a T-die (width: 900 mm, preset temperature: 220° C.), a chill roll (preset temperature: 120° C. to 130° C.), and a take-up unit.

(Preheating and First and Second Oblique Stretching Steps)

The polycarbonate resin film obtained as described above was subjected to a preheating treatment, first oblique stretching, and second oblique stretching with such apparatus as illustrated in FIG. 1 to FIG. 3 according to such clip pitch profile as illustrated in FIG. 5 to provide a retardation film. A specific procedure is as described below. First, the moving speeds of the left and right clips entering the preheating zone of the stretching apparatus were braked by high-torque sprockets arranged just in front of the preheating zone. Specifically, the clips moving in the holding zone A were braked by the sprockets to be moved to the preheating zone B. Next, the polycarbonate resin film (thickness: 150 μm, width (W₁): 765 mm) was preheated to 145° C. in the preheating zone of the stretching apparatus. In the preheating zone, the clip pitches (P₁) of the left and right clips were 125 mm. Next, the increase of the clip pitch of the right clips and the reduction of the clip pitch of the left clips were started simultaneously with the entry of the film into the first oblique stretching zone C. In the terminating portion of the first oblique stretching zone C, the change ratio (P₂/P₁) of the clip pitch of the right clips was 1.42, and the change ratio (P₃/P₁) of the clip pitch of the left clips was 0.72. It should be noted that the first oblique stretching was performed at 138° C. The width (W₂) of the film after the first oblique stretching was 1,092 ram (TD stretching ratio (W₂/W₁)=1.45 times). Next, simultaneously with the entry of the film into the second oblique stretching zone D, the increase of the clip pitch of the left clips was started, and the clip pitch was increased from P₃ to P₂. In the second oblique stretching zone D, the change ratio (P₂/P₃) of the clip pitch of the left clips was 1.97. Meanwhile, the clip pitch of the right clips was maintained at P₂ in the second oblique stretching zone D. It should be noted that the second oblique stretching was performed at 138° C. The width (W₃) of the film after the second oblique stretching was 1,419 mm. In addition, the stretching ratio (W₃/W₁) in the widthwise direction in the first oblique stretching step and the second oblique stretching step was 1.9 times.

(Releasing Step)

Next, in the releasing zone, the film was thermally fixed by being held at 125° C. for 60 seconds. The thermally fixed film was cooled to 100° C., and then the left and right clips were released.

Thus, the retardation film (thickness: 55 μm, width: 1,419 mm) was obtained. The resultant retardation film was subjected to the evaluations (1) and (2). The results are shown in Table 1.

Example 2

A retardation film was obtained in the same manner as in Example 1 except that in the film-stretching apparatus, sprockets rotationally driven at a constant speed were arranged instead of the high-torque sprockets. The resultant retardation film was subjected to the same evaluations as those of Example 1. The results are shown in Table 1.

Example 3

A retardation film was obtained in the same manner as in Example 1 except that in the film-stretching apparatus, the high-torque sprockets were arranged just in front of the preheating zone of the stretching apparatus, and sprockets rotationally driven at a constant speed were arranged in the first oblique stretching zone of the stretching apparatus. The resultant retardation film was subjected to the same evaluations as those of Example 1. The results are shown in Table 1.

Comparative Example 1

A retardation film was obtained in the same manner as in Example 1 except that none of a high-torque sprocket and a constant-speed rotating sprocket was arranged just in front of the preheating zone of the stretching apparatus (that is, the moving speeds of the left and right clips entering the preheating zone were not braked). The resultant retardation film was subjected to the same evaluations as those of Example 1. The results are shown in Table 1.

TABLE 1 Means for braking Alignment Initial clip moving speed angle preset (arrangement Initial After 1 Variation value position) stage hour after 1 hour Example 1 48° High-torque 48° 48° ±0.5° sprocket (just in front of preheating) Example 2 48° Constant-speed 48° 48° +0.3° rotating sprocket (just in front of preheating) Example 3 48° High-torque 48° 48° ±0.2° sprocket (just in front of preheating) Constant-speed rotating sprocket (stretching) Comparative 48° None 48° 52° ±1.0° Example 1

<Evaluation>

As shown in Table 1, according to the production method of the present invention, the alignment angle (direction of the slow axis) can be prevented from deviating from the preset value with time over the predetermined time period from the initial stage of the stretching by arranging means for braking the moving speeds of the clips in the range of from the holding zone to the oblique stretching zone (e.g., just in front of the preheating zone or at plural points in the area including just in front of the preheating zone and in the oblique stretching zone). Further, in addition to the effect, the variation of the alignment angle can be suppressed by using sprockets to be rotated at a constant speed as the braking means (i.e., by not only braking the clips entering the preheating zone but also controlling their speeds).

INDUSTRIAL APPLICABILITY

The retardation film obtained by the production method of the present invention is suitably used in a circularly polarizing plate, and as a result, is suitably used in an image display apparatus such as a liquid crystal display apparatus (LCD) or an organic electroluminescence display apparatus (OLED).

REFERENCE SIGNS LIST

10L endless loop

10R endless loop

20 clip

30 clip-carrying member

50 means for braking moving speed of clip

70 reference rail

90 pitch-setting rail

100 stretching apparatus

300 circularly polarizing plate

310 polarizer

320 first protective film

330 second protective film

340 retardation film 

The invention claimed is:
 1. A method of producing a retardation film, comprising: holding left and right end portions of a film with left and right variable pitch-type clips configured to have clip pitches changing in a longitudinal direction, respectively; preheating the film; performing at least one selected from the following (i) and (ii), while extending a distance between the left and right clips, to obliquely stretch the film: (i) making a change ratio of the clip pitch of the clips on one side in a traveling direction thereof and a change ratio of the clip pitch of the clips on another side in a traveling direction thereof different from each other, and (ii) making a position at which the clip pitch of the clips on the one side in the traveling direction starts to change and a position at which the clip pitch of the clips on the another side in the traveling direction starts to change different from each other; and releasing the clips holding the film, wherein moving speeds of the clips are braked immediately before the holding step and/or during a time period from the holding step to the oblique stretching step, and wherein the clips travel along rails which are symmetrical to each other.
 2. The method of producing a retardation film according to claim 1, wherein the braking of the moving speeds of the clips comprises applying a load to a link mechanism for changing the clip pitches to limit the clip moving speeds to predetermined speeds.
 3. A method of producing a retardation film, comprising: holding left and right end portions of a film with left and right variable pitch-type clips configured to have clip pitches changing in a longitudinal direction, respectively; preheating the film; performing at least one selected from the following (i) and (ii), while extending a distance between the left and right clips, to obliquely stretch the film: (i) making a change ratio of the clip pitch of the clips on one side in a traveling direction thereof and a change ratio of the clip pitch of the clips on another side in a traveling direction thereof different from each other, and (ii) making a position at which the clip pitch of the clips on the one side in the traveling direction starts to change and a position at which the clip pitch of the clips on the another side in the traveling direction starts to change different from each other; and releasing the clips holding the film, wherein moving speeds of the clips are braked immediately before the holding step and/or during a time period from the holding step to the oblique stretching step, and wherein the braking of the moving speeds of the clips comprises limiting the clip moving speeds to predetermined speeds through continuous or intermittent constant-speed rotation of an apparatus for braking the moving speeds of the clips.
 4. The method of producing a retardation film according to claim 1, wherein the braking of the moving speeds of the clips is performed at least once during a time period from a time point immediately before the holding step to the oblique stretching step, and is performed at least once during a time period from a time point immediately after the oblique stretching step to the releasing step.
 5. The method of producing a retardation film according to claim 1, wherein the oblique stretching step comprises increasing the clip pitch of the clips on the one side and reducing the clip pitch of the clips on the another side, while extending the distance between the left and right clips, to obliquely stretch the film, and maintaining or reducing the clip pitch of the clips on the one side and increasing the clip pitch of the clips on the another side so that the clip pitches of the left and right clips are equal to each other, while extending the distance between the left and right clips, to obliquely stretch the film.
 6. The method of producing a retardation film according to claim 5, wherein in the first oblique stretching step, the reducing the clip pitch of the clips on the another side is started after the increasing the clip pitch of the clips on the one side is started.
 7. The method of producing a retardation film according to claim 5, wherein in the first oblique stretching step and the second oblique stretching step, an oblique stretching ratio determined from the following equation (1) is 2.0 or more, and in the first oblique stretching step, a change ratio of the clip pitch of the clips on the another side is 0.5 or more and less than 1: S=√{square root over (W ₃ ²+v3′*( t3−t3 ))²)}/W ₁   Equation(1) where: W₁ represents a width in meters of the film before the first oblique stretching; W₃ represents a width in meters of the film after the second oblique stretching; v3′ represents a moving speed in meters per second of the clips having a clip pitch to be increased in the first oblique stretching step when the clip pitch of the clips changes to a predetermined clip pitch in the second oblique stretching step; t3 represents a time period in seconds from entry of the clips having a clip pitch to be reduced in the first oblique stretching step into a preheating zone to an end of the second oblique stretching step; and t3′ represents a time period in seconds from entry of the clips having a clip pitch to be increased in the first oblique stretching step into the preheating zone to the end of the second oblique stretching step.
 8. The method of producing a retardation film according to claim 5, wherein in the first oblique stretching step, a product of a change ratio of the clip pitch of the clips on the one side and a change ratio of the clip pitch of the clips on the another side is from 0.7 to 1.5.
 9. The method of producing a retardation film according to claim 1, wherein a formation material for the film contains a polycarbonate resin, a polyvinyl acetal resin, a cycloolefin-based resin, a cellulose-based resin, a cellulose ester-based resin, a polyester-based resin, or a polyester carbonate-based resin.
 10. A retardation film, which is produced by the production method of claim 1, the retardation film having an elongated shape and having a slow axis in a direction forming a predetermined angle relative to a lengthwise direction.
 11. A method of producing a circularly polarizing plate, comprising: obtaining a retardation film having an elongated shape by the production method of claim 1; and continuously bonding the resultant retardation film having an elongated shape and a polarizing plate having an elongated shape with lengthwise directions thereof aligned with each other while conveying the film and the plate.
 12. A film-stretching apparatus, comprising left and right variable pitch-type clips that pass a preheating zone and an oblique stretching zone in the stated order while holding left and right end portions of a film to be stretched, and that each show a change in clip pitch in a longitudinal direction in association with running movement, the apparatus being configured to perform at least one selected from the following (i) and (ii), while extending a distance between the left and right clips, to obliquely stretch the film in the oblique stretching zone: (i) making a change ratio of the clip pitch of the clips on one side in a traveling direction thereof and a change ratio of the clip pitch of the clips on another side in a traveling direction thereof different from each other, and (ii) making a position at which the clip pitch of the clips on the one side in the traveling direction starts to change and a position at which the clip pitch of the clips on the another side in the traveling direction starts to change different from each other, and the apparatus comprising at least one means for braking moving speeds of the clips in a range of from a zone in which the film is held to the oblique stretching zone, wherein the apparatus further comprises left and right rails which are symmetrical to each other, and wherein the left and right clips travel along the left and right rails.
 13. The film-stretching apparatus according to claim 12, wherein the apparatus comprises at least one means for braking the moving speeds of the clips in the range of from the zone in which the film is held to the oblique stretching zone, and comprises at least one means for braking the moving speeds of the clips in a range of from a position just behind the oblique stretching zone to a zone in which the clips holding the film are released. 